The main advantage of repeaterless transmission systems lies in the absence of electrically-powered elements in-line, thereby making such systems very reliable.
In order to increase the range of such systems, i.e. to increase the length of the transmission line, a known solution consists in using remote optical amplification. This is done by sending a beam of light known as the pump beam (e.g. at 1.48.mu.m) from the receiver terminal towards a section of doped fiber (e.g. using erbium) that is situated several tens of kilometers upstream along the transmission line. The pump beam activates the section of doped fiber which then amplifies the signal conveying the data, thus enabling it to reach the receiver terminal at a level that is sufficient.
To implement such remote optical amplification, the receiver terminal includes a pumping module which comprises a pump beam source and a wavelength multiplexer enabling the pump beam to be injected into the transmission line.
An improvement to that known solution consists in combining said remote optical amplification (or remote amplification) with local optical amplification (in the receiver terminal). This additional optical amplification makes it possible to increase the length of line separating the remote section of doped fiber from the receiver terminal, and thereby to increase the range of the link.
Generally, the increase in the optical power budget is about 2.5 dB over the basic configuration (i.e. when there is no local optical amplification in the receiver terminal), thereby making it possible to increase the range of the link.
Known systems implementing such remote optical amplification are described, in particular, in the following articles:
401 km, 622 Mbit/s and 357 km, 2.488 Gb/s IM/DD repeaterless transmission experiments using erbium-doped fiber amplifiers and error correcting code, by P. M. Gabla, J. L. Pamart, R. Uhel, E. Leclerc, J. O. Frorud, F. X. Ollivier, and S. Borderieux, published in Photonics Technology Letters, Vol. 4, No. 10, October 1992; and PA1 Performance improvement of direct detection systems using local and/or long distance pumped fiber amplifiers, by A. Bjarklev, J. H. Polvsen, K. Rottwitt, O. Lumholt, T. Rasmussen, published in Fiber and Integrated Optics, Vol. 10, pp. 215-223, 1991.
In those known systems, the (local) additional optical amplification is obtained using an additional optical amplifier placed downstream from the remote pumping module. A major drawback of the additional optical amplifier is that, of itself, it constitutes equipment that makes the receiver terminal more complex, and thus less reliable and more expensive.